1. Field of the Invention
The present invention relates to a stereo/monaural detection apparatus for detecting whether an audio input is a stereo signal or a monaural signal.
2. Description of the Prior Art
There is known a technique of detecting whether a two-channel audio input is a stereo signal or a monaural signal and selectively controlling the signal processing and so forth for the audio input in accordance with the result of such detection.
It is generally noted in the recent trend that an input audio signal is so processed as to expand the sound field thereof for enhancing the acoustic reality. Such sound field expansion needs to be selectively executed in a manner to change the signal processing depending on whether the audio input is stereo or monaural. A variety of sound field expansion systems are broadly classified into a stereo system for expanding the sound field of a stereo input signal and a monaural system for expanding the sound field of a monaural input signal by processing the same to a false stereo signal or a signal of enhanced acoustic reality. The conventional circuit based on such stereo sound field expansion system functions properly with regard to a stereo audio input signal in principle, but fails to achieve a sufficient sound expansion effect in case the audio input is a monaural signal. To the contrary, when a stereo audio signal is input to the known monaural sound field expansion circuit (e.g. for a false stereo signal), there may occur some incongruous aural impression. Since different processes need to be executed individually in conformity with a stereo or monaural input audio signal, it is urgently required to contrive a stereo/monaural detection circuit which is capable of discriminating a stereo signal and a monaural signal from each other with certainty.
In case an audio input is a telecast signal, discrimination between a stereo signal and a monaural signal can be performed by detecting the presence or absence of a stereo pilot signal superimposed selectively in a broadcasting station. However, when a stereo pilot signal is superimposed despite a monaural audio signal as in a case where monaural sound is inserted partially in a stereo program, the audio input is regarded as a stereo signal due to detection of such stereo pilot signal, and therefore an improper sound field expansion is executed on the basis of the erroneous discrimination result. Furthermore, such stereo pilot signal is utilizable merely with regard to a telecast signal and is not applicable to an audio signal obtained from any other source such as a video tape recorder, a video disc or an audio disc.
For achieving the above detection from an audio input itself, there may be contrived a means of detecting a stereo or monaural signal by discrimination of the level difference between two-channel input audio signals. However, there exists a problem that even when the audio input is stereo, the signal levels of the left and right channels may become equal to each other in case the acoustic image is positioned at the center, whereby it is not distinguishable from a monaural signal. And another disadvantage is existent with regard to instability of the discrimination when the input signal is feeble.